Klystron amplifier



Dec. 15, 1959 w. E. WATERS 2,917,655

KLYSTRON AMPLIFIER Filed Dec. 27, 1957 SIGNAL AMPLIFIED INPUT SIGNAL OUTPUT BUNGHER GATGHER & 4

SIGNAL AMPLJFIED INPUT SIGNAL OUTPUT INVENTOR Wu. LIAM E. WA rms 2,917,656 Patented Dec. 15,1959

R 2,917,656 KLYSTRON AMPLIFIER William E. Waters, Kensington, Md., assignor to the United States of America as represented by the Secretary of the Army Application December 27, 1957, Serial No. 705,720

1 Claim. (Cl. 315-5) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to microwave tubes in general, and more particularly to an improved klystron amplifier.

The object of this invention is to provide an improved klystron amplifier having a power gain which is considerably greater than that of the conventional klystron amplifier.

The conventional two-cavity klystron amplifier basically comprises a source of electrons such as a hot cathode, an electron gun for accelerating and focusing the electrons into a beam, a buncher cavity which is excited by the microwave R-F input signal to be amplified and through Whose grids the beam is passed, a drift space following the buncher cavity, a catcher cavity excited by the beam of electrons passing through its grids, and a collector electrode upon which the beam terminates. The R-F voltage produced between the grids of the buncher cavity by the R-F input signal velocity-modulates the electron beam so as to produce bunching of the beam, that is, causing an R-F component of current to appear in the beam at points beyond the buncher grids. The length of the drift space-that is, the distance between buncher and catcher grids-is chosen so that maximum bunching occurs at the catcher grids, resulting in the production of a microwave R-F field in the catcher cavity. By suitable coupling devices, power may be extracted from the catcher cavity. Since, for operation at the fundamental frequency, it is possible to extract more power from the catcher cavity than is used to excite the buncher cavity, the device acts as an amplifier of the R-F excitation power.

In a klystron amplifier structure as described above, it is conventionally assumed, and the structure is built to insure, that the D.-C. potential between the cavities (that is, in the drift space) is independent of position. The beam current, the cavity voltages, and the tube dimensions are chosen so that space charge due to the presence of the electrons is of very low order, making possible the neglecting of any potential lowering effect in the drift space caused-by the space charge.

It has been known that the presence. of space charge is detrimental in reflex klystron oscillators. While studying these detrimental effects of space charge in the reflex klystron, I discovered that the presence of a space charge potential distribution in the drift space of a two-cavity klystron amplifier, on the other hand, is not detrimental, but rather, may result in a considerable increase in power gain. A theoretical discussion of space charge effects in klystrons and the possible increases in power gain by making use of a space charge potential distribution in the two-cavity klystron amplifier is presented in an article published by me in the IRE Transactions on Electron Devices, January 1957, pp. 49-58.

In the present invention, the power gain of a twocavity klystron amplifier is greatly increased by purposely designing the tube so that an appreciable: space charge potential distribution is present in the drift space. It has been found that this can result in much greater bunching so that considerably more power can be extracted by the buncher cavity for a given input, thereby increasing the power gain. Under a given set of conditions, the power gain may be increased by as much as 16 decibels. Or, a given power output can be obtained with smaller power input.

The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a two-cavity klystron amplifier in accordance with the invention.

Fig. 1A is a graph of the potential in the drift space of the device of Fig. 1.

Fig. 2 is a schematic diagram of another embodiment in accordance with the invention.

Like numerals designate like parts throughout the drawing. In Fig. l, a two-cavity klystron amplifier is shown having improved means in accordance with the invention. A heater element 12 heats a cathode 15 to provide a source of electrons. An electron gun represented by the accelerating electrode 16 accelerates and focuses the electrons into a beam 20. The cathode 15 is connected to a negative voltage source B- and the electrode 16 is connected to an accelerating potential E in a conven tional manner.

The electron beam 20 passes through the grids 25a of a conventional form of buncher cavity 25 serving as the input cavity. The buncher cavity 25 is excited by the input signal to be amplified by means of an input coupling loop 32. This produced an R-F voltage at the buncher cavity grids 25a which velocity modulates the electron beam 20 passing therethrough. After passing the buncher cavity grids 25a, the electron beam 20 passes into a drift space 40, and then passes through the grids 35a of a conventional form of catcher cavity 35 serving as the output cavity. Energy is coupled out of the catcher cavity 35 bymeans of an output loop 34. After passing through the catcher cavity grids 35a, the electron beam 20 terminates on a collector electrode 48. The buncher cavity 25, the catcher cavity 35, and the collector electrode 48, are preferably grounded for safe handling.

The description so far applies to the conventional two-cavity klystron amplifier which is well known in the art. In the conventional klystron amplifier, the cathode voltage B, the length of the drift space (that is, the distance between buncher and catcher grids 25a and 35a), and the signal input, are coordinated so that maximum bunching occurs at the catcher grids 35a. The structure is conventionally designed to insure that the D.-C. potential in the drift space 40 is constant and that any space charge effects are negligible. This is ordinarily accomplished by constructing the drift space enclosure of a close fitting metallic tube surrounding the electron beam; the metallic tube walls tend to cancel out the effects of space charge.

I have discovered from theoretical calculations and experimental data that considerably increased power gain can be obtained in a two-cavity klystron amplifier by causing a space charge potential distribution to appear in the drift space 40. The reason for this appears to be that considerably better bunching characteristics are obtained for a Wider range of input voltages so that more matically illustrated in Fig. 1, the potential distribution in the drift space 40 for an idealized tube being graphically illustrated in Fig. 1A. As can be seen from Fig. 1A, the potential in the drift space 40 slowly falls to a minimum value Vm at about the midpoint between the'bunch- .er and catcher grids 25a and 35a. This is the same type of distribution that occurs in an equivalent plane diode and :may be determined by well known theoretical relation- .ships. The drift space diameter, as determined by the diameter of the walls 51, is chosen to be several beam diameters so that these walls 51 do not tend to cancel :out space charge efifects, as does the close-fitting tube of the conventional klystron amplifier.

Although it .is theoretically possible to obtain the space charge in .the drift space 40 illustrated in Fig. 1, a practical difiiculty arises because the potential depression caused by the space charge tends to trap positive ions produced by collisions between the beam electrons and residual gas molecules. This reduces the extent and magnitude of the potential depression and tends to defeat its purpose. To overcome this practical difiiculty, an open grid-60 may be placed at or near the potential minimum, and adjusted to a-value of about Vm (see Fig. 1A) which would be the idealized potential minimum to be expected in "the absence of gas molecules. The effect of this grid would be to remove ions as they are formed.

A second way of producing the desired space charge potential distribution in the drift space 40 is schematically illustrated in Fig. 2. A series of grids 61, 62 and 63 having voltages Vm, Vm and Vm', respectively, are arranged to approximate the desired space charge potential distribution in the drift space 40. These grids 61, 62 and 63 also serve to sweep out ions as they accumulate. This second way has the advantage that the desired potential distribution may be obtained even in the absence of a considerable amount of space charge. It has the disadvantage however of being somewhat more complex because of the additional grids and the additional voltage sources needed.

Once the space charge potential distribution is established, as in either Figs. 1 or 2, the cathode voltage B-, the length of the drift space, and the signal input, are coordinated in the conventional manner so that maximum bunching occurs at the catcher cavity grids 35a. The improvement in power gain obtained by the presence of a space charge distribution increases as the potential minimum Vm at the midpoint between the cavity grids 25a and 25b decreases; that is, as the effective space charge present increases. This increase in power gain :continues with increased effective space charge until instability occurs. Theoretical calculations have shown that the power gain of two-cavity klystron amplifiers in accordance with the present invention may be increased by as much as 16 decibels before instability occurs. It should be noted that this invention may also be applied to klystron amplifiers having more than two cavities.

In atypical embodiment of Fig. 1, an electron beam 4 20 is focused to a diameter of 6 millimeters with a maximum beam current density of about 65 milliamperes per square centimeter, an easily obtainable figure. The cathode voltage B- is set at 1000 volts, and the open grid is set at about -250 volts. These values are chosen to provide a natural space charge in the drift space and maximum bunching at the catcher grids 35a for the length of drift space employed. Power gains up to 16 decibels greater than that of a conventional two-cavity klystron amplifier were readily obtained from this device.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claim.

I claim as my invention:

An improved two-cavity klystron amplifier comprising in combination: a source of electrons, an electron gun for accelerating and focusing the electrons into an electron beam, a buncher cavity having grids through which said beam is passed, means for exciting said buncher cavity with the R-F input signal to be amplified, :said R-F input signal producing an R-F voltage between the grids of said buncher cavity thereby velocity modulating said electron beam, a drift space through which said beam passes after passing through the grids of said buncher cavity, said drift space having a diameter which is at least several beam diameters so as to permit the formation of an appreciable space charge therein, the magnitude and diameter of said beam current being chosen to produce an appreciable space charge field throughout said drift space, an open grid located substantially at the potential minimum produced by said space charge, said grid having a voltage substantially equal to the voltage of the potential minimum which would be ideally produced by said space charge in the absence of gas molecules, said grid serving only to remove ions as they are formed and thereby maintain the space charge in said drift space, a catcher cavity having grids through which said beam passes after leaving said drift space, the bunching of the electrons in said beam exciting said catcher cavity, the length of said drift space being chosen for maximum bunching at the grids of said catcher cavity, means for coupling out energy from said catcher cavity, and a collector electrode upon which said beam terminates.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,611 Samuel Aug. 13, 1946 2,408,809 Pierce Oct. 8, 1946 2,442,662 Peterson June 1, 1948 2,455,269 Pierce Nov. 30, 1948 2,468,928 Hansen et al. May 3, 1949 2,527,600 Touraton et al. Oct. 31, 1950 FOREIGN PATENTS 51,548 France June 29, 1942 

